The present invention generally relates to a method for producing a high quality useful layer of semiconductor material on a substrate. In particular, the method includes implanting at least two different atomic species into a face of a donor substrate to a controlled mean implantation depth to form a weakened zone therein and to define a useful layer. The implanting step is conducted to minimize low-frequency roughness at the weakened zone. Next, a support substrate is bonded to the face of the donor substrate, and the useful layer is detached along the weakened zone. A structure is thus formed that includes the useful layer on the support substrate with the useful layer presenting a surface for further processing. The technique also includes thermally treating the structure to minimize high-frequency roughness of the surface of the useful layer. The result is a surface having sufficient smoothness so that chemical mechanical polishing (CMP) is not needed.
Implanting of species is understood to mean any bombardment of atomic or ionic species, likely to introduce such species onto the material of the implanted substrate, with a maximum concentration of the implanted species situated at a preset depth from the substrate relative to the bombarded surface. Such processes, such as the SMART-CUT® type process are known. Details concerning the SMART-CUT® process can be found in the document ‘Silicon-On-Insulator Technology: Materials to VLSI, 2nd Edition’, by Jean-Pierre Colinge from Kluwer Academic Publishers, pages 50 and 51.
Such processes advantageously produce structures comprising a thin layer of semiconductor material. These structures can be an SOI-type (Silicon On Insulator) structure. The structures produced by such processes are used for applications in the fields of microelectronics, optics and/or optronics.
The specifications concerning the state of the surface of the structures used in these fields are generally very strict. The roughness of the thin layer is a parameter which to a certain extent affects the quality of the components which will be created on the structure. The roughness is generally expressed either by an average quadratic value known as Root Mean Square (RMS), or by a Power Spectral Density (PSD) measurement. For example, it is typical to find roughness specifications which must not exceed 5 Angströms in RMS value.
Roughness measurements can be accomplished by using an atomic force microscope AFM (Atomic Force Microscope). With this type of instrument, the roughness is measured on surfaces swept by the point of the AFM microscope, ranging from 1×1 μm2 to 10×10 μm2, and more rarely 50×50 μm2, or 100×100 μm2.
Roughness can be characterized, in particular, according to two modalities. First, the roughness can be described as being high frequency and corresponds to small-sized swept surfaces (on the order of 1×1 μm2). Second, the roughness can be described as being low frequency and corresponds to larger size swept surfaces (on the order of 10×10 μm2, or more). The “high frequencies” and “low frequencies” thus correspond to spatial frequencies associated with the roughness measurements. The example specification of 5 Angströms RMS given above is therefore a low-frequency roughness, corresponding to a swept surface of 10×10 μm2.
Low roughness at high frequencies is particularly desirable to obtain good gate oxides. Low roughness at low frequencies (or waviness) is of interest particularly when the aim is to bond another substrate onto the free surface of the thin layer.
The thin layers which are obtained by known transfer processes (for example by using the SMART-CUT® type process) have surface roughness values that are generally greater than the specifications mentioned above, in the absence of applying a specific treatment to the surface of the thin layer. Finishing steps are therefore generally utilized on the structure following the detachment stage, to improve the quality of the thin transferred layer. More precisely, it is an object of these finishing steps to smooth out the roughness characteristics, including both high-frequency and low-frequency roughness. Such finishing steps generally make use of operations such as chemical-mechanical polishing (CMP), kiln annealing, rapid annealing, oxidation/deoxidation sequences and the like.
A first type of known process for decreasing surface roughness includes chemical-mechanical polishing of the free surface of the thin layer. This type of process effectively reduces the roughness of the free surface of the thin layer by decreasing all waviness, especially those at the lowest frequencies. However, chemical-mechanical polishing causes defects (for example, cold working defects) in the thin layer. In addition, it compromises the uniformity of the free surface of the thin layer (in particular uniformity according to very low frequencies). These disadvantages may be further aggravated in the case where it is important to polish the surface of the thin layer.
An alternative to such CMP polishing is to treat the surface of the structure by annealing in a hydrogenated atmosphere. U.S. Pat. No. 6,362,076 describes a finishing process that enables the roughness of a thin layer, transferred according to a SMART-CUT® type process, to be decreased. The finishing process includes, following a detachment step, a thermal annealing step in a reduced atmosphere containing hydrogen. Thermal annealing in a reduced atmosphere containing hydrogen enables smoothing out of the high-frequency roughness by surface reconstruction. But this treatment does not totally eliminate the lowest-frequency roughness (5 to 10 micrometers).
The published French document FR 2 797 713 describes combining two different types of surface treatments, each treatment acting on a distinct roughness frequency range. Rapid thermal annealing in a reducing atmosphere as well as chemical-mechanical polishing (CMP) are carried out, wherein these steps are distinguished by their effects on different frequency ranges. In the case of such treatment, annealing in a reducing atmosphere is beneficial for smoothing the high-frequencies roughness (a spatial period of less than 3 Angströms), but is less efficient for reducing waviness, which are the low frequency roughness parameters. Chemical-mechanical polishing is effective for decreasing the low-frequency waviness.
As discussed, chemical-mechanical polishing is intrinsically associated with certain disadvantages. Therefore, the known processes for improving the quality of the thin layer of a structure of the type described above include limitations and/or drawbacks.